Recent Advances in the Chemistry of Polyurethanes

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Abstract
Major emphasis in recent years has been placed upon efforts to develop aliphatic isocyanates to impart light stability and improved stability toward hydrolysis. Another major area of research activity were new methods for the generation of isocyanates. Aliphatic isocyanates. The first commercial aliphatic diisocyanate used was 1,6—hexamethylene diisocyanate (HDI), OCN (CH2)6NCO, a colorless liquid, boiling at 127° C at 10 mm Hg; it is less reactive than either tolylene diisocyanate (TDI) or 4,4′-diisocyanatodiphenylmethane (MDI). However, in the presence of certain metal catalysts, such as tin, lead, bismuth, zinc, iron, cobalt, etc., the activity of the aliphatic isocyanate groups is equal to or greater than that of the isocyanate groups in TDI. The use of hexamethylene diisocyanate (HDI) leads to urethane polymers having better resistance to discoloration, hydrolysis, and heat degradation as compared to TDI. Due to its high vapor pressure and toxicity, a higher molecular weight modification of HDI is often employed which is made by reaction of 3 moles of HDI with 1 mole of water:

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